Planning to shed a few pounds after all the holiday excess is over? Better check your carbon footprint.

Contrary to beliefs held even by nutritionists, physicians, physical trainers and people who should know better, lost weight does not leave our bodies as heat, get converted into muscle or get passed out in the feces. It largely leaves the body in the form of carbon dioxide exhaled from our lungs, according to an article published Tuesday in the British Medical Journal.

Losing weigh requires unlocking the carbon stored in fat cells, and lungs "are the primary excretory organ for fat," write Ruben Meerman and Andrew J. Brown of Australia's University of New South Wales.

When someone sets a goal of losing weight without losing lean muscle, she is in effect planning to metabolize the triglycerides stored in her adipocytes, or fat cells. Chemically speaking, triglycerides must be oxidized, or broken down into their component parts. Oxidizing 10 kilograms of fat will take 29 kg of inhaled oxygen, and will produce 28 kg of carbon dioxide (CO2) and 11 kg of water (H2O).

"Losing weight requires unlocking the carbon stored in fat cells, thus reinforcing that often heard refrain of 'eat less, move more,'" the authors of the primer on weight loss wrote. Given what they called the "widespread misconceptions" about weight loss, the biochemistry of the process ought to be taught in high schools and university biochemistry courses.